(1) Field of the Invention
The present invention relates to a technique for wireless communication using high frequency, and in particular to the structure of a high frequency circuit that amplifies a high frequency signal.
(2) Description of the Related Art
With the rapid spread of mobile telephones in recent years, techniques relating to high frequency devices and high frequency circuits have advanced dramatically. Such advances are followed year by year with improved performance and reduced size and cost of wireless communication devices such as mobile telephones.
In addition, with the integration of conventional computer terminals and mobile telephones, wireless LAN (local area network) systems that perform high speed data communication are also rapidly entering into widespread use. Such wireless LANs are one type of broadband communication, and currently support high speed data transfer of several hundred kilobits to several tens of megabits per second, using a frequency band of approximately 2 GHz to 5 GHz. Such broadband communication enables distribution of high-definition moving images and the like.
The structure of a high frequency circuit in a wireless communication device used in a wireless LAN system is approximately the same as that of a high frequency circuit in a mobile telephone. FIG. 1 shows the structure of a conventional high frequency circuit. An antenna unit 5 is composed of antennas 51 and 52, and an antenna switch 53. This is a diversity structure that uses a lateral antenna that is highly sensitive. The transmission/reception switch 9 switches the antenna unit 5 to be connected to a transmission circuit or a reception circuit. The transmission/reception switch 9 is necessary in time division duplex (TDD), which is generally employed in wireless LAN systems to use the same frequency for both transmission and reception. A BPF (bandpass filter) 43 used for reception reduces unnecessary band components in signals received by the antenna unit 5. An LNA (low-noise amplifier) 12 amplifies an input signal, so that the amplified signal is low in noise, and outputs the amplified signal to a down mixer. A PA (power amplifier) 11 amplifies, to high output, a signal sent from an up mixer. BPFs 41 and 42 reduce unnecessary band components before and after the PA, respectively.
Details about wireless LANs can be found in “Musen Akusesu Gijutsu (Wireless Access Technologies)”, Journal of IEICE, February 2001, pp. 105-111. Furthermore, details about antenna diversity, and the structure of the filter and the transmission/reception unit can be found in Journal of IEICE, November 2001, pp. 775-789 and pp. 796-802.
Since there are instances in which a high power signal is input into the LNA in the high frequency circuit of a wireless communication device used in a wireless LAN system, it is necessary to use an amplifier with a large saturation output to prevent signal distortion due to amplification by the LNA in nonlinear areas. As a result, amplifiers with approximately the same output are used for the LNA and the PA.
The expense of active elements such as the LNA and the PA contributes to high costs of high frequency circuits. Furthermore, the component size of high-output amplifiers is a hindrance in producing circuits that can be mounted compactly.